1. Field of the Invention
The present invention relates to high speed diazonium salts useful in diazo-type photoreproduction, and more particularly, to such salts having improved thermal stability, and producing prints which have stable backgrounds and improved color.
2. Description of the Prior Art
In diazo-type photoreproduction, a suitable base material, such as paper or transparent film, is coated with a sensitizing composition containing the diazo sensitizer and thereafter exposed to actinic radiation, e.g., UV light, through a suitable pattern which may be a line drawing, photographic transparency, etc. The photochemical sensitivity of the diazo compounds typically employed resides in the near ultraviolet region of the spectrum, and is centered around 400 nm. Thus, the diazo compound present in those areas of the sensitized layer affected by the exposure radiation, i.e., unprotected by the image portions of the copy being reproduced, undergoes a decomposition reaction whereby its capacity to combine subsequently with the coupling component to form dyestuff is destroyed.
In the semi-dry or two component diazotype process, the coupling component or components are included as ingredients of the sensitizing composition. Thus, development is effected merely by subjecting the element thus exposed to an alkaline atmosphere, e.g., by bringing it into contact with ammonia vapor, to neutralize the acidic inhibitor with the concomitant formation of dyestuff in the non-exposed areas due to coupling of the residual diazo compound and color coupling components. A reproduction of the original pattern is obtained as an azo dye image on a light background. The above procedure can be effectively carried out with the utilization of automatic printing and processing equipment specifically provided for such purposes, such apparatus being capable of performing successively the dual operations of exposure and dry development. Usually, apparatus of this type is provided with physically separate chambers through which the exposed print material is successively transported, via suitable mechanical means.
As will be recognized, it is of critical importance, regardless of the particular process being employed, that the print obtained possess satisfactory contrast, density, dye brightness, background clarity, etc. Thus, those areas of the light-sensitive material subjected to the actinic effects of the exposure radiation should be totally devoid of spurious discoloration. Hence, it is readily understandable that it is highly desirable that the light-sensitive diazonium compound should, ideally, yield colorless products upon light-induced decomposition.
However, many of the light-sensitive diazo compounds previously suggested for such use fail to provide optimum contrast levels, the failures in this regard being traceable to the tendency of the diazonium compound to undergo decomposition due to side reactions, with the concomitant formation of colored products. In this connection, reference may be made to the tendency of many diazonium compounds to decompose upon inadvertent heating. In other instances, it is found that a given diazo-coupler system exhibits an intolerable level of precoupling or autocoupling. Regardless of the particular mechanism responsible for such deleterious effects, the production of photocopy having the requisite sensitometric quality proves highly problematical.
Because of the long exposure times typically required with diazo type photoreproduction, attempts have been made in the prior art to prepare so-called "high speed diazos" which work considerably faster than those known previously. For example, British Pat. No. 919,037 and U.S. Pat. No. 3,397,058, issued Aug. 13, 1968 to van Loon et al disclose diazonium compounds of the general formula: ##STR2## wherein R.sub.1 and R.sub.2 are alkyl, cycloalkyl, or aralkyl groups or substituted alkyl, cycloalkyl, or aralkyl groups or form together with the nitrogen atom a heterocyclic ring; R.sub.3 is a phenyl or substituted phenyl group; R.sub.4 is a hydrogen atom or a substituent group; and X is a chloride or sulphate ion, or zinc chloride double salt, or cadmium chloride double salt radical. Included within the compounds described therein are those in which R.sub.3 is a phenyl or substituted phenyl group with the substituents being halogen atoms, alkoxy groups, methyl groups, and mono- and dialkylamino groups. R.sub.4 in the above formula is stated to include a halogen atom, a methyl group, or an alkoxy group. Specific compounds disclosed include those where R.sub.1 and R.sub.2 are both carbon chains of one to three carbons, or in the alternative one or both of R.sub.1 and R.sub.2 can be a benzene group, or R.sub.1 and R.sub.2 can be combined as a morpholino or piperidino group. An example is also included in which R.sub.4 is phenoxy.
U.S. Pat No. 3,338,713, issued Aug. 29, 1967 to Hendrick et al discloses diazonium compounds of the formula ##STR3## wherein X is an anion; R.sub.1 is an alkyl, cycloalkyl or aralkyl radical; --CO--R.sub.2 is an acyl group; R.sub.3 is an acylamino, alkylsulphonamido, aralkylsulphonamido, arylsulphonamido, or triazinyl group; and R.sub.4 is an alkyl, branched alkyl, cycloalkyl, aralkyl, or aryl radical. Included are compounds in which R.sub.4 is p-chlorophenyl.
U.S. Pat. No. 3,463,639, issued Aug. 26, 1969 to Baltazzi, discloses compounds which can be diazotized to yield light sensitive diazonium salts of the formula ##STR4## wherein A represents alkoxyl, aroxyl, acyloxyl, hydroxyl, halogen, alkyl, aryl or aralkyl; B is a secondary amino residue; and R.sub.1 is lower alkyl, cycloalkyl, aryl, aralkyl or lower alkyl-substituted heterocyclic radical; R.sub.2 is alkyl, cycloalkyl, aryl or aralkyl; and ##STR5## is a saturated cyclic amino radical derived from aziridine, pyrrolidine, lower alkyl-substituted pyrrolidone, piperidine, lower alkyl-substituted piperidine, hexamethyleneimine, azabicyclononane, piperazine, lower alkyl-substituted piperazine, morpholine and lower alkyl-substituted morpholine. Included is a compound in which A is phenoxyethoxyl.
U.S. Pat. No. 3,719,491, issued March 6, 1973 to Mizianty discloses high speed diazo type reproduction materials suitable for both one-component and two-component diazo type reproduction processes including a diazonium salt of the formula: ##STR6## wherein R.sup.1 and R.sup.2 are selected from alkyl, aralkyl, and the atoms necessary to form a heterocyclic ring with the amino nitrogen atom; R.sup.3 is selected from hydrogen, alkoxy, and halogen; n is an integer of up to 4; and Y is an anion. The salts are stated to be especially suited to two-component diazo reproduction processes utilizing ammonia development. lt is stated therein that the thermal instability of the diazo material was reduced by including a cyano group, without appreciably affecting the high speed reproduction properties.
British Pat. No. 919,037 and U.S. Pat. No. 3,397,058 teach only the use of 4-aminobenzene diazonium salts bearing 3-phenoxy or substituted 3-phenoxy groups, without mentioning the ammonia induced discoloration found upon development (often referred to as pinking), or of background discoloration instability upon print aging. The compounds of Baltazzi suffer from thermal instability. Mizianty is principally concerned with devising a high speed diazo system without sacrificing thermal stability. The only mention of a clean background in Mizianty is at column 11, line 30 and column 12, line 4 with no cogniscence that the observation is in any way connected with any singular property of the structure described.
In spite of these previous preparations of so-called high speed diazos, commercial products based upon such compounds have not been successful. This is believed to be largely due to their thermal instability, which is often combined with either poor dye stability to acid and light, or the predispositon background (or minimum density) (D.sub.min) of prints made using such compounds to rapidly discolor to an objectionable pink color.
Consequently, although the diazos described in the above patents provided high speeds of reproduction, a need continues to exist for high speed diazonium salts which possess thermal stability, good dye stability to acid and light, provide a background or minimum density (D.sub.min) which is resistant to discoloration, and develop rapidly as well as allowing a wider range of azo dye colors, especially a more neutral blue color with B.O.N. arylamides, such as ##STR7##
Moreover, although many of the diazo compounds promulgated provide some margin of advantage, it is invariably found that improvement in a given property, e.g., thermal stability, is accompanied by other undesirable effects, e.g., suppression in light-sensitivity, development speed, and the like.
Thus, the provision of light-sensitive diazonium compounds with superior resistance to the formation of unwanted discoloration of print background, without adversely affecting other essential properties, continues to challenge diazotype technology.
Thus, a primary object of the present invention resides in the provision of light-sensitive diazonium compounds wherein the disadvantages discussed above are eliminated or at least mitigated to a substantial extent.
Another object of the present invention resides in the provision of diazo sensitizing compositions capable of providing image reproduction substantially devoid of background discoloration.